1. Field of the Invention
This invention relates to an electric power steering unit to be used for a vehicle and, more particularly, it relates to a field of technology effectively applicable to electric power steering gears to be used for rack and pinion type steering systems.
2. Related Art Statement
In recent years, most vehicles are equipped with a so-called power steering gear, and various type power steering unit, such as a hydraulically or electrically operated type power steering unit has been designed to assist the steering power of the vehicle. In these electric power steering units, as one for applicable to rack and pinion type steering systems, a unit which the steering assisting power is obtained by an electric motor arranged coaxially with a rack-shaft, as Japanese Patent Application Laid-Open No. 8-98451, is known.
Such an electric power steering unit shown in FIG. 6, comprises an electric motor 52 arranged coaxially with a rack-shaft 51 so that the steering assisting power generated by the electric motor 52 is transmitted to the rack-shaft 51 by way of a ball screw mechanism 53. Then, the guiding wheels of the vehicle can be turned by utilizing both the manual steering power of the driver and the steering assisting power.
The rack-shaft 51 is linked to the guiding wheels (which hereinafter may simply be referred to as "the wheels") typically by way of tie rods or knuckle arms arranged at the respective opposite ends thereof and also linked to the steering column 54 that is coupled to the steering wheel (which hereinafter may be referred to as "the handle") by way of a rack and pinion gear so that it may be reciprocatively moved in the horizontal directions of FIG. 6 as the steering operation by the driver. The electric motor 52 has a cylindrical yoke 55 containing coaxially therein a cylindrical armature shaft 56 and a field device 57 and is fed with power from a power supply section 58. The field device 57 comprises magnets 59 arranged on the inner peripheral portion of the yoke 55 and an armature core 60 arranged on the outer peripheral portion of the armature shaft 56. The rotary power generated by the electric motor 52 is transmitted to the rack-shaft 51 by way of a ball screw mechanism 53 arranged at the left end of the armature shaft 56 in FIG. 6. Note that the armature shaft 56 is supported at a right side portion thereof by an angular bearing 65 held within housing 61.
The ball screw mechanism 53 has a well known constitution, which comprises a large number of balls 64 arranged between a nut section 62 and a screw section 63 thereof, the nut section 62 being press-fit into and caulked against the armature shaft 56. With this arrangement, the rotary power of the electric motor 53 is transmitted to the rack-shaft 51 by way of the nut section 62, the balls 64 and the screw section 63 to produce axial reciprocative power there, which is then used to assist the steering power.
A power steering unit having the above described configuration is then mounted to the vehicle by securely fitting the housing 61 and the yoke 55 on the right side of FIG. 6 to the vehicle main body. More specifically, the housing 61 is secured to the vehicle by screw engagement by way of a fitting section 69 integrally formed with it. On the other hand, the yoke 55 is fitted to the vehicle by holding the pipe-shaped end (on the left side in FIG. 6) of the yoke 55 under a bracket 70 having a U-shaped cross section at a middle portion thereof. Note that an elastic member (not shown) typically made of rubber is arranged around the pipe-shaped end with a small diameter of the yoke 55 so that the yoke 55 is held under the U-shaped area of the bracket 70 with the elastic member disposed therebetween.
Now, the wheels may be turned to extremes during a servicing operation or when the steering wheel is turned while the vehicle is standing still. Then, some of the stoppers (not shown) of the tie rods provided at the both ends of the rack-shaft 51 may abut the corresponding end of the housing 61 and the yoke 55 to resist the turning motion of the wheels. Under this condition, the housing 61 and the yoke 55 are subjected to a large impact that can exceed several tons as a result of the abutment. Particularly, the vehicle maybe jacked up during a servicing operation and driven to run seemingly at a very high speed that can not be achieved under any ordinary driving conditions. Since each of the related members of the vehicle is subjected to a very large power in such a case, they are normally designed to bear an impact as large as ten tons.
Meanwhile, as some of the stoppers abut the corresponding end of the housing, the rotating part of the armature shaft 56 is also stopped and the rack-shaft 51 is subjected to a large impact due to the force of inertia. Thus, the armature shaft 56 is also subjected to a reaction force exerted by the ball mechanism 53.
If the power transmission path is looked into in such a case, it will be found that, when a stopper hits the left side end of the yoke 55 in FIG. 6, the yoke 55 is firstly and directly subjected to a compressive force. On the part of the electric motor 52, on the other hand, it then exerts the force of inertia onto the rack-shaft 51, trying to move further to the right relative to the rack-shaft 51 and the armature shaft 56 is consequently subjected to a tensile force, which is the reaction force exerted to it by the rack-shaft 51. This tensile force is then transmitted back to the stopper by way of the angular bearing 65, the housing 61 and the yoke 55.
When another stopper hits the right side end of housing 61 in FIG. 6, the housing 61 is directly subjected to a compressive force. On the other hand, the rack-shaft 51 is subjected to a force trying to move it further left due to the force of inertia of the electric motor 52 and the armature shaft 56 is consequently subjected to a compressive force, which is the reaction force exerted to it by the rack-shaft 51. This compressive force is then transmitted back to the stopper by way of the angular bearing 65 and the housing 61.
As described above, the armature shaft 56 of conventional power steering units is required to have a strength that can sufficiently make it withstand severe operating conditions because not only the housing 61 and the yoke 55 but also the armature shaft 56 are subjected to large tensile and compressive forces in operation. Thus, it has to have a large thickness and be made of a material highly resistant to external forces to consequently reduce its manufacturing efficiency and raise its manufacturing cost.
Additionally, as shown in FIG. 6, the yoke 55 and the armature shaft 56 have very complicated respective profiles that pose on them highly sophisticated requirements in terms of the material, the manufacturing process and the processing precision to make them have a satisfactory strength. Consequently, the cost of manufacturing such an armature shaft and a yoke will have to be further raised.
An object of the present invention is to provide an electric power steering unit having an armature shaft that is free from axial forces and hence may have less rigorous requirements for its strength so that it may be manufactured more efficiently at reduced cost.
Another object of the present invention is to provide an electric power steering unit having an armature shaft and a yoke having simplified respective profiles that pose on them less sophisticated requirements in terms of the material, the manufacturing process and the processing precision to make them show a satisfactory strength so that such an armature shaft and a yoke may be manufactured at reduced cost.
The above-described and other objects, and novel feature of the present invention will become apparent more fully from the description of the following specification in conjunction with the accompanying drawings.